Electroluminescence (EL) is the emission of light under electric-field excitation. Based on this mechanism, EL lamps and displays are finding an increasing number of applications in the field of flat panel displays due to the growing demand for portable computers, communication equipment, and consumer electronic products. EL lamps also provide uniform light emission independent of viewing angle and they are insensitive to mechanical shock and vibration. They can be easily dc-driven at 1.5–9 volts by using inverters that generate ac voltages of about 100–300 V (peak-to-peak) at frequencies of 50 to 1000 Hz.
The two major EL lamp constructions are generally referred to as thin-film and thick-film. Thin-film EL lamps are made by depositing alternating thin layers of dielectric materials, phosphors and conductive oxides on a glass substrate using a vapor deposition technique such as CVD. By contrast, thick-film lamps are made by suspending powdered materials in resinous materials and then applying the materials in layers onto a plastic film using conventional screen printing techniques. Hence, the thick-film EL lamps can be thin, flexible and rugged thereby making them suitable for a wider range of lighting applications.
A cross-sectional illustration of a conventional thick-film EL lamp is shown in FIG. 1. The lamp 2 has two dielectric layers 20 and 22. A first conductive material 4, such as aluminum or graphite, coated on a plastic film 12b forms a first electrode of the lamp 2; while a thin layer of a transparent conductive material 6, such as indium tin oxide, coated on a second plastic film 12a forms a second electrode. Sandwiched between the two conductive electrodes 4 and 6 are two layers 20 and 22 of dielectric material 14 which can be, for example, cyanoethyl cellulose or cyanoethyl starch. Adjacent to the first electrode 4 is a layer of dielectric material 14 in which are embedded particles of a ferroelectric material 10, preferably barium titanate. Adjacent to the second electrode 6 is a layer of dielectric material 14 in which are embedded particles of an electroluminescent phosphor 8. The phosphors available for thick-film EL lamps are primarily comprised of zinc sulfide that has been doped with various activators, e.g., Cu, Au, Ag, Mn, Br, I, and Cl. Examples of these phosphors are described in U.S. Pat. Nos. 5,009,808, 5,702,643, 6,090,311, and 5,643,496. Typically, the individual particles of the EL phosphors are encapsulated with an inorganic coating in order improve their resistance to moisture-induced degradation. Examples of such coatings are described in U.S. Pat. Nos. 5,220,243, 5,244,750, 6,309,700, and 6,064,150.
For accent lighting applications, signage, multi-color information displays and the like, it is desirable to provide EL lamp manufacturers with a wide range of emission colors to choose from. Moreover, it is desirable to provide single-component phosphors for each color rather than blends since the different phosphors in the blends will tend to degrade at different rates causing the emission color to shift. Unfortunately, the range of emission colors of EL phosphors tends to be somewhat limited. In particular, the color choices are heavily weighted towards the blue to green region of the visible spectrum with fewer choices available in the yellow to red region.
Zinc sulfide electroluminescent (EL) phosphors co-activated with manganese and copper ions (ZnS:Mn,Cu) are well known. Examples of these phosphors and their methods of manufacture are described in U.S. Pat. Nos. 4,859,361, 5,009,808, and 6,682,664. When stimulated by an electric field in a conventional thick-film electroluminescent lamp, these phosphors exhibit an orange-yellow emission with an x color coordinate of at least 0.520. However, single-component, yellow-emitting electroluminescent phosphors having an emission with an x color coordinate less than 0.510 are not known. Thus, it would be an advantage to be able to provide lamp manufacturers with a yellow-emitting EL phosphor.